It is known to provide an electronic article surveillance (EAS) tag device which is responsive to both a first frequency, which is in the microwave range, and a second, lower frequency. Such a tag device is disclosed in U.S. Pat. No. 4,736,207, issued to Siikarla et. al. and commonly assigned with the present application. The disclosure of that U.S. Pat. No. 4,736,207 is incorporated herein by reference.
The tag device described in the '207 patent includes in compact form an antenna which is tuned to receive the first and second frequencies. The tag is used with detecting equipment that emits signals at the first and second frequencies, and in the presence of such signals the tag device receives the two signals via the antenna, and in effect mixes the two signals to provide a signal at the first frequency modulated by the second frequency. The tag reradiates the mixed signal, and the reradiated signal is received by the detecting equipment to detect the presence of the tag device.
As disclosed in the above-referenced Siikarla et al. patent, the conventional tag includes a thin, rectangular and planar substrate made of an insulative material. A conductive layer is formed on one surface of the substrate in a predetermined pattern so as to provide most of the circuit elements required for the desired dual-frequency antenna. The antenna is completed by means of a diode mounted so as to connect two portions of the conductive layer. Siikarla et al. also teach that an additional insulative layer is overlaid on the conductive antenna layer, while leaving access to the conductive layer so that the tag may be deactivated by applying an energy pulse to destroy the diode.
Although dual-frequency tags have proved quite useful for their intended purposes, it would be advantageous if deactivation of such tags could be performed without directly coupling the tag to a source of an electrical pulse. U.S. Pat. No. 5,257,009, issued to Narlow (which has an assignee and an inventor in common with this application) discloses a dual-frequency tag that can be deactivated by exposing the tag to an electrostatic field which changes the capacitance of a variable capacitor that is associated with the tag's antenna circuit. It would be desirable to provide a field-deactivatable dual-frequency tag that is easier to manufacture than the tag disclosed in the '009 patent.